Curtain coating is a coating process in which a linear flow of a liquid coating composition is deposited on a surface of a moving substrate, such as paper web. The coating composition forms a liquid sheet that falls freely before impinging onto the moving substrate that is to be coated.
Curtain coating is a technique well suited for coating a paper web with a liquid barrier layer, since the barrier substance must form a film that covers the surface entirely, or as close to entirely as possible. However, the curtain coating process is susceptible to certain defects, such as irregular heel formation, air entrainment and curtain instabilities that may result in a coated surface that displays smaller or bigger areas that lacks coating. In order to discover such defects in the coverage of the coating, which can be very small due to e.g. air bubbles in the liquid coating, a test may be made by applying a coloured test liquid onto the coated surface. Any defects will then appear as coloured areas or so called pinholes. For some applications it is particularly important that the coating does not contain any defects or at least very few defects, for example in the food industry and for liquid packages (i.e. packages containing liquids).
It is previously known that polyvinyl alcohol is a good barrier against fat, solvents and gases. A coating comprising polyvinyl alcohol deposited on a substrate is known to be suitable as a barrier against fat, oxygen, odours, aromas, etc. in for example packaging material based on paperboard substrate.
In curtain coating a suitable dynamic surface tension, calculated from Mach-angle measurements in the falling curtain, is required in order for the free falling curtain of coating liquid to form a stable curtain and not to have defects in the form of holes. Typically, for Newtonian fluids, the coating liquid should have a dynamic surface tension of less than 40 mNm to satisfy the criteria of a falling curtain without the formation of holes. However, polyvinyl alcohol does have a considerably higher surface tension which leads to an instable curtain at low flow rates.
In order to reduce the surface tension of polyvinyl alcohol it has been proposed to add a surface active substance, a surfactant, to the polyvinyl alcohol, for example as described in EP 2182113, using a surfactant of the type Air Products EnviroGem AE03. However, it has been found that even though the stability of the curtain improved when using a surfactant, the barrier effect of the coating deteriorated, and to such a degree that it was not usable for producing liquid packages. As other examples, it has been proposed to add relatively large amounts of ethanol or similar alcohol to the polyvinyl alcohol, but this is however not a satisfying solution of the coating problem in the paper industry since it requires handling and recycling of resulting volatile organic compounds (VOC).
To summarise, in order to use curtain coating to obtain a barrier on a substrate, such as a polyvinyl alcohol barrier on a fibre substrate, e.g. paperboard, it is necessary to use added substances that reduces the dynamic surface tension in order to have a stable process, but these substances will at the same time have a negative effect on the film formation and therefore the barrier will be defective.